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Abstract:
Isostructural orthorhombic oxides of the general formula (Mo,V,Te,Sb,Nb,Ta)O-x are an important class of solids, which are interesting as catalysts for oxidation of light alkanes. We investigated relations between the electronic properties of MoV-oxide (orthorhombic M1 phase) and its catalytic performance in the oxidation of ethane, propane, and n-butane. Operando conductivity and permittivity measurements were performed and complemented by near-ambient-pressure X-ray photoelectron spectroscopy. In contrast to the n-type MoVTeNb-oxide, MoV-oxide showed p-type semiconducting behavior. The conductivity of the sample adapted sensitively to the surrounding atmosphere, not only to alkane chain lengths but also to reactant conversion levels. However, no measurable change in band bending depending on the alkane chain length was observed, indicating that the gas-phase-dependent surface potential barrier, which controls the charge transfer between reactants and catalyst, is less pronounced or missing in dry alkane oxidation feeds. The addition of steam in propane oxidation led to a decrease of its conductivity and work function. Steam significantly influenced the surface layer on MoV-oxide, resulting in an enrichment of covalently bonded V5+ species and surface hydroxylation. A small change in the surface potential barrier induced by wet propane oxidation feed can contribute to a modification of the bulk-surface charge transfer and improved selectivity to acrylic acid.
